Spectroscopic Scatterometer System

ABSTRACT

Before the diffraction from a diffracting structure on a semiconductor wafer is measured, where necessary, the film thickness and index of refraction of the films underneath the structure are first measured using spectroscopic reflectometry or spectroscopic ellipsometry. A rigorous model is then used to calculate intensity or ellipsometric signatures of the diffracting structure. The diffracting structure is then measured using a spectroscopic scatterometer using polarized and broadband radiation to obtain an intensity or ellipsometric signature of the diffracting structure. Such signature is then matched with the signatures in the database to determine the grating shape parameters of the structure.

BACKGROUND OF THE INVENTION

This invention relates in general to scatterometers and in particular,to a spectroscopic scatterometer system.

As the integration and speed of microelectronic devices increase,circuit structures continue to shrink in dimension size and to improvein terms of profile edge sharpness. The state-of-the-art devices requirea considerable number of process steps. It is becoming increasinglyimportant to have an accurate measurement of submicron linewidth andquantitative description of the profile of the etched structures on apattern wafer at each process step. Furthermore, there is a growing needfor wafer process monitoring and close-loop control such asfocus-exposure control in photolithography.

Diffraction-based analysis techniques such as scatterometry areespecially well suited for microelectronics metrology applicationsbecause they are nondestructive, sufficiently accurate, repeatable,rapid, simple and inexpensive relative to critical dimension-scanningelectron microscopy (CD-SEM).

Scatterometry is the angle-resolved measurement and characterization oflight scattered from a structure. For structures that are periodic,incident light is scattered or diffracted into different orders. Theangular location θ_(r) of the m^(th) diffraction order with respect tothe angle of incidence θ_(i) is specified by the grating equation:$\begin{matrix}{{{\sin\quad\theta_{1}} + {\sin\quad\theta_{r}}} = {m\quad\frac{\lambda}{d}}} & (1)\end{matrix}$where λ is the wavelength of incident light and d the period of thediffracting structure.

The diffracted light pattern from a structure can be used as a“fingerprint” or “signature” for identifying the dimensions of thestructure itself. In addition to period, more specific dimensions, suchas width, step height, and the shape of the line, the thickness of theunderlay film layers, and angle of the side-walls, referred to below asparameters of the structure, can also be measured by analyzing thescatter pattern.

Since the periods of the gratings in the state-of-the-art devices aregenerally below 1 μm, only the 0^(th) and +/−1^(ST) diffraction ordersexist over a practical angular range. A traditional scatterometer thatmeasures the entire diffraction envelope does not provide the datarequired for an accurate analysis. One prior optical technique forcharacterizing submicron periodic topographic structures is called 2-Θscatterometry.

The 2-Θ scatterometer monitors the intensity of a single diffractionorder as a function of-the angle of incidence of the illuminating lightbeam. The intensity variation of the 0^(th) as well as higherdiffraction orders from the sample provides information which is usefulfor determining the properties of the sample which is illuminated.Because the properties of a sample are determined by the process used tofabricate the sample, the information is also useful as an indirectmonitor of the process.

In 2-Θ scatterometry, a single wavelength coherent light beam, forexample, a helium-neon laser, is incident upon a sample mounted on astage. By either rotating the sample stage or illumination beam, theangle of incidence on the sample is changed. The intensity of theparticular diffraction order (such as zeroth-order or first order) as afunction of incident angle, which is called a 2-Θ plot or scatter“signature” is then downloaded to a computer. In order to determine thedifferent parameters such as linewidth, step height, shape of the line,and angle of the side-walls (the angle the side-wall makes with theunderlying surface, also known as the “wall angle”), a diffraction modelis employed. Different grating parameters outlined above areparameterized and a parameter space is defined by allowing eachgrating-shaped parameter to vary over a certain range.

A rigorous diffraction model is used to calculate the theoreticaldiffracted light fingerprint from each grating in the parameter space,and a statistical prediction algorithm is trained on this theoreticalcalibration data. Subsequently, this prediction algorithm is used todetermine the parameters that correspond to the 2-Θ plots or scatter“signature” measured from a target structure on a sample.

While 2-Θ scatterometry has been useful in some circumstances, it hasmany disadvantages. The periodic diffracting structure is frequentlysituated over one or more films that transmit light. Therefore, anydiffraction model employed must account for the thicknesses andrefractive indices of all films underneath the diffracting structure. Inone approach, the thickness and refractive indices of all layers must beknown in advance. This is undesirable since frequently, these quantitiesare not known in advance. In particular, the film thickness and opticalindices of materials used in semiconductor fabrication often vary fromprocess to process.

Another approach to solve the above problem is to include all unknownparameters in the model, including film thickness and optical indices ofunderlying film materials. By thus increasing the number of variables inthe model, the number of signatures that has to be calculated increaseexponentially, so that the computation time involved renders suchapproach inappropriate for real-time measurements.

Furthermore, since the intensity of the particular diffraction order asa function of incidence angle of the sampling beam is acquiredsequentially as the incident angle is varied, data acquisition for the2-Θ plot or scatter “signature” is slow and the detected intensity issubject to various noise sources such as system vibration and randomelectronic noise which may change over time as the incident angle isvaried.

Another approach is proposed by Ziger in U.S. Pat. No. 5,607,800. Inthis approach, where the measurement of a particular patterned film isdesired, a first patterned arrangement having predetermined and knowngrating characteristics close to those of the patterned film to bemeasured is first made, such as by forming a line-and-space pattern on afirst wafer. A spectroreflectometer is then used to measure theamplitude of reflected signals from such first arrangement to obtain asignature. Then a second patterned arrangement having known gratingcharacteristics close to those of the patterned film to be measured,such as another line-and-space pattern on a second wafer, is obtainedand a spectroreflectometer is used to measure the amplitude of reflectedsignal from such arrangement to obtain a second signature. The processis repeated on additional wafers and the signatures so formed areorganized as a database. Then, the target pattern film of the sample ismeasured using a spectroref lectometer and its signature compared tothose present in the database. The signature in the database thatmatches the signature of the target film is then used to find thegrating characteristics or parameters of the target film.

Ziger's approach is limited and impractical, since it requiresreplication of multiple reference patterns analogous to the targetpattern and measurements of such reference patterns to construct adatabase before a measurement can be made of the target pattern. Ziger'sapproach also requires contrast difference between the reflectivity ofthe film versus the reflectivity of the substrate. In other words,Ziger's method cannot be used to measure the grating characteristics online patterns made of a material having a reflectivity similar to thatof the underlying substrate.

None of the above-described approaches is entirely satisfactory. It istherefore desirable to provide an improved scatterometer with betterperformance.

SUMMARY OF THE INVENTION

One aspect of the invention is directed towards a method of measuringone or more parameters of a diffracting structure on an underlyingstructure, said underlying structure having a film thickness and anoptical index, comprising providing an optical index and a filmthickness of the underlying structure; constructing a reference databaseof one or more parameters related to said diffracting structure usingsaid optical index and film thickness of the underlying structure; anddirecting a beam of electromagnetic radiation at a plurality ofwavelengths at said diffracting structure. The method further comprisesdetecting intensities or ellipsometric parameters at said plurality ofwavelengths of a diffraction from said structure; and comparing saiddetected intensities or ellipsometric parameters to said database todetermine said one or more parameters.

Another aspect of the invention is directed towards an apparatus formeasuring one or more parameters of a diffracting structure on anunderlying structure, said underlying structure having a film thicknessand an optical index, comprising means for constructing a referencedatabase of one or more parameters related to said diffracting structureusing an optical index and a film thickness of the underlying structure;and means for directing a beam of electromagnetic radiation includingenergy at a plurality of wavelengths at said diffracting structure. Theapparatus further comprises means for detecting intensities orellipsometric parameters of a diffraction from said structure at saidplurality of wavelengths; and means for comparing said detectedintensities or ellipsometric parameters to said database to determinesaid one or more parameters.

Another aspect of the invention is directed towards a scatterometer formeasuring a parameter of a diffracting structure of a sample, includinga source which emits broadband radiation; a polarizer that polarizes thebroadband radiation to produce a sampling beam sampling the structure;and means for detecting intensities or ellipsometric parameters of adiffraction from the structure over a range of wavelengths.

An additional aspect of the invention is directed towards a method formeasuring one or more parameters of a diffracting structure of a sample,including providing broadband radiation; polarizing the broadbandradiation to produce a sampling beam; and directing the sampling beamtowards the structure. The method further comprises detecting radiationof the sampling beam that has been diffracted from the structure over arange of wavelengths; and comparing the detected radiation to areference to determine said one or more parameters.

One more aspect of the invention is directed towards an instrument formeasuring one or more parameters of a diffracting structure on anunderlying structure of a sample, comprising a source of broadbandradiation; a polarizer polarizing said radiation to provide a samplingbeam towards the sample; and an analyzer for receiving diffractedradiation from the structure to provide an output beam. The instrumentfurther comprises a spectrometer detecting the output beam.

One more aspect of the invention is directed towards a method formeasuring one or more parameters of a diffracting structure on anunderlying structure of a sample, comprising performing spectroscopicmeasurements on the underlying structure to determine itscharacteristics; constructing a reference database of one or moreparameters related to said diffracting structure using characteristicsof the underlying structure; and performing scatterometric measurementson the diffracting structure to obtain intensity or ellipsometric data;and comparing said intensity or ellipsometric data to the referencedatabase to derive said one or more parameters.

Yet another aspect of the invention is directed towards an instrumentfor measuring a sample, comprising a spectroscopic device measuring filmthickness data, and index of refraction data of the sample over aspectrum; a scatterometer measuring diffraction data from a diffractingstructure of said sample over a spectrum and means for deriving physicalparameters related to the structure from the film thickness data, indexof refraction data, and diffraction data.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic view of a spectroscopic scatterometer toillustrate the preferred embodiment of the invention.

FIG. 1B is a schematic view of a portion of the spectroscopicscatterometer of FIG. 1A to illustrate the preferred embodiment of theinvention.

FIG. 2 is a cross-sectional view of a semiconductor wafer including aline pattern of photoresist on a bare silicon substrate useful forillustrating the invention.

FIG. 3A is a graphical plot of the intensity of the zeroth diffractionorder as 51 different functions of the angle of incidence of theilluminating light beam in a 2-Θ scatterometer, where the nominallinewidth is assumed to be 250 nanometers, and the 51 functions areobtained assuming linewidths from 225 to 275 nanometers, at 1 nanometersteps, for comparison with predicted results of the invention.

FIG. 3B is a graphical plot of the intensity of the zeroth diffractionorder as 51 different functions of the wavelength of the illuminatinglight beam according to the invention where the nominal linewidth isassumed to be 250 nanometers, and the 51 functions are obtained assuminglinewidths from 225 to 275 nanometers, at 1 nanometer steps, forcomparison with predicted results of the invention.

FIG. 3C is a plot of the means square error difference measurement as afunction of linewidth,. between the signature generated for the gratinghaving the nominal linewidth of 250 nanometers and other signaturesobtained for other linewidths using 2-Θ scatterometry, and using thepreferred embodiment of this invention over a full range of the spectrumand over UV and visual wavelength bands of the full spectrum useful forillustrating the invention.

FIG. 4A is a graphical plot of the intensity of an ellipsometricparameter tan(psi) as 5 different functions of the wavelength of theilluminating light beam according to the invention where the nominallinewidth is assumed to be 180 nanometers, and the 5 functions areobtained assuming linewidths at 178, 179, 180, 181, 182 nanometers, forcomparison with predicted results of the invention.

FIG. 4B is a graphical plot of the intensity of an ellipsometricparameter cos(delta) as 5 different functions of the wavelength of theilluminating light beam according to the invention where the nominallinewidth is assumed to be 180 nanometers, and the 5 functions areobtained assuming linewidths at 178, 179, 180, 181, 182 nanometers, forcomparison with predicted results of the invention.

FIG. 5 is a plot of two sets of correlation functions between thesignature for the grating having the nominal linewidth of 180 nanometersand other signatures for gratings at other linewidths, one set obtainedusing tan(psi) and the other set obtained using cos(delta).

For simplicity in description, identical components are identified bythe same numerals in this application.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

This invention is based on the recognition that, by measuring orotherwise obtaining characteristics such as the film thickness andoptical index of the underlying films underneath the diffractingstructure, the subsequent tasks of construction of a database andmatching a signature of the diffracting structure to the database aremuch simplified. Furthermore, if spectroscopic ellipsometry is used tomeasure the film thickness and optical index of the underlying film(s)under the diffracting structure, an instrument which can be used forspectroscopic ellipsometry as well as for spectroscopic scatterometrymay be provided for carrying out both functions. In the preferredembodiment, the spectroscopic ellipsometer and its associatedspectroscopic scatterometer in the instrument may share many commonoptical elements to reduce the cost of the combined instrument andsimplify its operation.

By first measuring the film thickness and optical refractive index ofthe underlying films, one no longer needs to include such parameters inthe computation of the model or database and subsequent matching ofsignatures that much simplifies the computation task.

FIG. 1A is a schematic view of a spectroscopic scatterometer system toillustrate the preferred embodiment of the invention. As shown in FIG.1A, system 10 advantageously combines features of a spectroscopicscatterometer, spectroscopic ellipsometer and spectroscopicreflectometer. The spectroscopic reflectometer or the spectroscopicellipsometer may be used for measuring the film thickness and refractiveindex of the underlying structure underneath the diffracting structure.As shown in FIG. 1A, a semiconductor wafer may comprise a siliconsubstrate 12 a, a film 12 b on the substrate and a diffracting structure12 c such as a photoresist pattern on the film, where the film is atleast partially light-transmissive and has a certain film thickness andrefractive index (n and k, the real and imaginary components of theindex).

Before the diffracting structure 12 c is measured, an XYZ stage 14 isused for moving the wafer in the horizontal XY directions in order tofirst measure the film thickness and refractive index of the underlyingstructure underneath the photoresist pattern 12 c. Stage 14 may also beused to adjust the z height of the wafer 12 as described below. Stage 14moves the wafer to a position as shown in FIG. 1B so that the samplingbeam of radiation illuminates a portion of film 12 b away from structure12 c. In reference to FIGS. 1A, 1B, for the purpose of measuring thefilm thickness and refractive index of the underlying structure ( 12 band 12 a), a broadband radiation source such as white light source 22supplies light through a fiber optic cable 24 which randomizes thepolarization and creates a uniform light source for illuminating thewafer. Preferably, source 22 supplies electromagnetic radiation havingwavelengths in the range of at least 230 to 800 nm. Upon emerging fromfiber 24, the radiation passes through an optical illuminator that mayinclude a slit aperture and a focus lens (not shown). The slit aperturecauses the emerging light beam to image a small area of layer 12 b. Thelight emerging from illuminator 26 is polarized by a polarizer 28 toproduce a polarized sampling beam 30 illuminating the layer 12 b.

The radiation originating from sampling beam 30 that is reflected bylayer 12 b, passed through an analyzer 32 and to a spectrometer 34 todetect different spectral components of the reflected radiation. In thespectroscopic ellipsometry mode of system 10 for measuring filmthickness and refractive index, either the polarizer 28 or the analyzer30 is rotated (to cause relative rotational motion between the polarizerand the analyzer) when spectrometer 34 is detecting the reflected lightat a plurality of wavelengths, such as those in the spectrum of theradiation source 22, where the rotation is controlled by computer 40 ina manner known to those skilled in the art. The reflected intensities atdifferent wavelengths detected is supplied to computer 40 which computesthe film thickness and n and k values of the refractive index of layer12 b in a manner known to those skilled in the art. For a description ofa spectroscopic ellipsometer, please see U.S. Pat. No. 5,608,526, issuedMar. 4, 1997.

While spectroscopic ellipsometry may be preferred for measuring filmthickness and refractive index, in some applications where there mayonly be one or two relatively thick films underneath the diffractingstructure, a spectroscopic reflectometer (also known asspectroreflectometer and spectrophotometer) may be adequate formeasuring the film thickness and refractive index. For this purpose,lens 23 collects and directs radiation from source 22 to a beam splitter52, which reflects part of the incoming beam towards the focus lens 54which focuses the radiation to layer 12 b. The light reflected by thelayer 12 b is collected by lens 54, passes through the beam splitter 52to a spectrometer in the spectroscopic reflectometer 60. The spectralcomponents at different wavelengths measured are detected and signalsrepresenting such components are supplied to computer 40 for determiningthe film thickness and refractive index in a manner described, forexample, in U.S. patent application Ser. No. 08/227,482, filed Apr. 14,1994. Spectroscopic devices other than the spectroscopic reflectometerand spectroscopic ellipsometer may also be used for measuring the filmthickness and refractive index of layer 12 b and are within the scope ofthe invention. An example of such spectroscopic devices include the n &k Analyzer of n & k Technology Inc. of Santa Clara, California, anddescribed in “Optical Characterization of Amorphous and PolycrystallineSilicon Films,” by Ibok et al., reprinted from August 1995 edition ofSolid State Technology published by PennWell Publishing Company;“Optical Dispersion Relations for Amorphous Semiconductors and AmorphousDielectrics,” by Forouhi et al., Physical Review B, vol. 34, no. 10, pp7018-7026, Nov. 15, 1986; “Optical Properties of CrystallineSemiconductors and Dielectrics,” by Forouhi et al., Physical Review B,vol. 38, no. 3, pp 1865-1874, Jul. 15, 1988 and U.S. Pat. No. 4,905,170.

For the purpose of adjusting the height of wafer 12 relative to thepolarizer 28, analyzer 32 to achieve proper focus in the spectroscopicellipsometry measurement, or relative to the focus lens 54 andspectroscopic reflectometer 60 in the spectroreflectometer measurement,the height of the wafer may need to be adjusted by means of stage 14prior to the measurement. For this purpose, a portion of the radiationreflected by layer 12 b (or layer 12 c in the, description that follows)and collected by lens 54 is reflected by a beamsplitter 62 towards afocusing and pattern recognition block 64 for comparing the reflectedimage to a pattern. Block 62 then sends information concerning thecomparison to computer 40 which controls stage 14. Stage 14, in turn,moves the wafer 12 up or down in the vertical or Z direction in order tomove wafer 12 to a proper height relative to the optical components ofsystem 10.

Once the film thickness and refractive index of the one or more filmsunderneath the diffracting structure 12 c have been so measured, areference database may now be constructed by means of computem 40. Wherethe film thickness and refractive index of the one or more filmsunderneath the diffracting structure 12 c are known to begin with, orcan be estimated, it is possible to omit the step of measuring thesequantities. To construct the reference database, characteristicsconcerning the diffracting structure 12 c may be parameterized and theparameters database is defined by allowing an unknown grating parameterof the structure, such as linewidth, height and wall angle to vary overa certain range. This is illustrated by reference to FIG. 2.

FIG. 2 is a cross-sectional view of a semiconductor wafer comprising asilicon substrate 12 a and a diffracting structure 12 c′ having alinewidth CD, pitch p, height h, and wall angle α as shown in FIG. 2.Thus, the grating shape parameters that can be parameterized and variedover a certain range include CD, h and α. A rigorous diffraction model,such as the model method by modal expansion (MMME), is used to calculatethe theoretical diffracted light fingerprint from each grating in theparameter space, and a, statistical prediction algorithm such asPartial-Leased-Squares (PLS) or Minimum-Mean-Square-Error (MMSE) istrained on this theoretical calibration data. For a description of theMMME, please see “Convergence of the Coupled-wave Method for MetallicLamellar Diffraction Gratings,” by Li et al., Journal of the OpticalSociety of America A Vol. 10, No. 6, pp. 1184-1189, June 1993; and“Multilayer Modal Method for Diffraction Gratings of Arbitrary Profile,Depth, and Permittivity,” by Li et al., Journal of the Optical Societyof America A Vol. 10, No. 12, pp. 2582-2591, December 1993.

Instead of using the MMME, the grating shape parameters can also beparameterized using rigorous coupling waveguide analysis (“RCWA”). Suchmethod is described, for example, in “Rigorous coupled-wave analysis ofplanar-grating diffraction,” by M. Moharam et al., J. Opt. Soc. Am.,Vol. 71, No. 7, July 1981, pp. 811-818, “Stable implementation of therigorous coupled-wave analysis for surface-relief gratings: enhancedtransmittance matrix approach,” by M. Moharam et al., J. Opt. Soc. Am.A, Vol. 12, No. 5, May 1995, pp. 1077-1086, and “Analysis andApplications of Optical Diffraction by Gratings,” T. Gaylord et al.,Proceedinqs of the IEEE, Vol. 73, No. 5, May 1985, pp. 894-937.

Where more than one grating shape parameter is varied, the calculationof fingerprints may be performed by varying only one parameter at a timewhile keeping the other parameters at selected constant values withinselected ranges. Then another parameter is allowed to vary and so on.Subsequently, this prediction algorithm is used to determine the valuesof the parameters that correspond to the fingerprint measured from layer12 c′.

Since the film thickness and optical indices of any film underlyingdiffracting structure 12 c or 12 c′ are known from the spectroscopicellipsometry or spectroreflectometry measurements, or are otherwiseknown, these values may be used in construction of the referencedatabase so that the film thickness and refractive index need not beparameters in the database. This greatly reduces the number of variablesin the parameter space and also greatly reduces the number of signaturesthat need to be calculated for the reference database. Thus, compared tothe 2-Θ scatterometry method where such variables need to be taken intoaccount in the parameter space and the calculation of signatures, thisinvention enables a smaller database to be used when searching forsolutions. Furthermore, due to the number of variables that areparameterized in such 2-Θ scatterometry method, there may be multiplesolutions which causes difficulties in obtaining a correct solution. Byreducing the size of the database, this invention enables uniquesolutions to be found in most cases. In this manner, this inventionreduces the computation time by many orders of magnitude compared to 2-Θscatterometry.

The process for measuring the signature from layer 12 c and 12 c′ willnow be described in reference to FIG. 1A. As described above, stage 14moves wafer 12 so that the sampling beam 30 illuminates an area of theunderlying film 12 b without illuminating any portion of the diffractingstructure 12 c. Now in order to measure structure 12 c, the computer 40causes stage 14 to move the wafer along a direction in the XY plane sothat the sampling beam 30 impinges on layer 12 c (or 12 c′ in FIG. 2).Broadband radiation from source 22 is polarized by polarizer 28 into apolarized broadbeam sampling beam 30. A diffraction of beam 30 issupplied to spectrometer 34 which measures substantially simultaneouslythe radiation intensities at different wavelengths of a diffraction fromstructure 12 c, such as at wavelengths across the spectrum of radiationsource 22. In the preferred embodiment, the zeroth diffraction orderintensity is measured, although for some structures, measurement ofhigher diffraction order intensities may also be feasible. The processjust described is the scatterometric measurement mode of system 10.

Zeroth or higher diffraction order intensities at different wavelengthsdetected by spectrometer 34 are supplied to computer 40 for analysis anddetermination of a signature of structure 12 c or 12 c′. This signatureis then compared to those precomputed in the reference database in themanner described above. The grating shape parameters of the signature inthe reference database that matches the measured signature of structure12 c or 12 c′ are then the grating shape parameters of the structure.

In the scatterometric measurement mode, analyzer 32 may be simplyremoved from the optical path from structure 12 c to spectrometer 34.Alternatively, polarizer 28 and analyzer 32 may be controlled by meansof computer 40 so that polarizer 28 passes radiation of a certainpolarization and analyzer 32 is oriented to pass radiation of the samepolarization as that passed by polarizer 28. This invention is based onthe discovery that, where the incidence plane of the beam 30 issubstantially normal to the grating 12 c, the sensitivity ofscatterometric measurements is improved if polarizer 28 is oriented tosupply a sampling beam 30 polarized in the TE mode (S-polarized) andanalyzer 32 is oriented to pass light in the TE mode. Alternatively,where the incidence plane of the beam 30 is substantially parallel tothe grating 12 c, the sensitivity of scatterometric measurements isimproved if polarizer 28 is oriented to supply a sampling beam 30polarized in the TM mode (P-polarized) and analyzer 32 is oriented topass light in the TM mode.

If more than one diffracting structure having different shape parametersare present on wafer 12, stage 14 may be controlled by computer 40 tomove wafer 12 so that the sampling beam 30 is directed towards each ofsuch diffracting structures one at a time. System 10 is then operated inthe scatterometric measuring mode to obtain signatures from each of suchdiffracting structures. The signature of each diffracting structure maythen be matched with a signature in the reference database in order toobtain the grating shape parameters of such structure. It will be notedthat, where measurement of the characteristics of the underlyingstructure (12 a, 12 b) is necessary, this will need to be performed onlyonce for each wafer and the reference database will need to beconstructed only once for the wafer as well. After these have beenaccomplished, the scatterometric measurements of the differentdiffracting structures on wafer 12 may be performed quickly and thesignatures of each diffracting structure matched to the referencedatabase expeditiously. As noted above, since the reference databasecontains a smaller number of signatures, the matching or predictionspeed of the grating shape parameters of the different diffractingstructures on wafer 12 is greatly increased. This makes real time andin-line measurements of the diffracting structures possible. In someapplications, a number of semiconductor wafers made by the same processhave the same underlying structure underneath the diffractionstructures; these underlying structures of the different wafers may havesubstantially the same film thicknesses and indices of refraction. Ifthis is the case, the above-described process for measuring filmthickness and index refraction and the construction of the referencedatabase may need to be performed only once for the entire batch ofwafers made by the same process, if the tolerance of the process isknown. This further speeds up the measurement and calculation process.

As compared to 2-Θ scatterometry, the spectroscopic scatterometer ofthis invention measures diffraction and a number of wavelengthssimultaneously. This is in contrast to 2-Θ scatterometry where the usertakes a measurement of the diffraction at one angle of incidence at atime. Such feature also speeds up the measurement process. It will alsobe noted that the above-described reference database is constructedwithout the use of reference samples. Thus, the user does not have tomake reference wafers having diffracting structures analogous to the onebeing measured or having to take measurements from such referencesamples before a database can be constructed. Furthermore, a rigorouslyradical model such as MMME is used to achieve accurate results.

Preferably, in the spectroscopic ellipsometry mode and thescatterometric measurement mode, sampling beam 30 is directed towardswafer 12 at an oblique angle to layer 12 b and 12 c. Sampling beam 30 ispreferably at an oblique angle in the range of 40 to 80°, and morepreferably in the range of 60 to 80° for measurement of silicon wafers,from a normal to the layers on the wafer 12. A particularly preferredangle of incidence from the normal is about 76° which is substantiallythe Brewster angle for silicon. In system 10, the spectroscopicellipsometer and spectroscopic scatterometer advantageously employ manycommon optical elements, such as the broadband source 22, fiber 24,illuminator 26, polarizer 28 and spectrometer 34. This simplifies thedesign of system 10, reduces cost and simplifies its operation.

The process for adjusting the height of wafer 12 relative to the opticalcomponents in the spectroreflectometry and spectroscopic ellipsometrymodes has been described above. However, when light reflected frombeamsplitter 52 is directed towards a diffracting structure such as 12c, it is preferable for the light so reflected to be polarized and tohave the same polarization as that in sampling beam 30 when the heightof the wafer 12 is adjusted. For this purpose,. radiation supplied bysource 22 is passed through a polarizer 72 before it is directed tobeamsplitter 52. The optical axis of polarizer 72 is controlled bycomputer 40 so that it has the same orientation as the optical axis ofpolarizer 28 when the focusing and pattern recognition block 64 is usedto detect radiation reflected from structure 12 c and stage 14 iscontrolled by computer 40 to adjust height of the wafer until it is atthe proper height relative to the sampling beam 30. Polarizer 72 doesnot affect the height adjustment process during the spectroreflectometryand spectroscopic ellipsometry modes or the spectroscopic reflectometrymeasurements. The polarized radiation detected by spectroscopicreflectometer 60 may also be used to normalize the intensity measurementin the scatterometer mode described above at an oblique angle to reducethe effects of intensity variations of source 22.

FIG. 3A is a graphical plot of the intensity of the zeroth diffractionorder as 51 functions of the angle of incidence of the illuminatinglight beam in a 2-Θ scatterometer measuring structure 12 c′ of FIG. 2,where the nominal linewidth is assumed to be 250 nm, and the 51.functions are obtained assuming linewidths from 225 to 275 nanometers,at 1 nanometer steps. The incidence angles used in a calculation of thegraphical plot in FIG. 3A varies from 0 to 60° with an uniform incrementof 1°, which results in 61 datapoints per signature curve. The lightbeam is assumed to be TE polarized and the wavelength was 0.6328microns.

FIG. 3B is a graphical plot of the intensity of zeroth diffraction orderas a function of the wavelength of the illuminating light beam accordingto the invention used for measuring structure 12 c′ of FIG. 2 where thenominal linewidth is assumed to be 250 nm, and the 51 functions areobtained assuming linewidths from 225 to 275 nanometers, at 1 nanometersteps. These 51 functions are obtained by means of the MMME model methodrigorous diffraction method described above, making use of the known ormeasured index of refraction and film thickness information. Thesecurves are used in comparison with measured results of the invention topredict linewidth of the measured structure. The intensity of the zerothorder is calculated as a function of the wavelength of the illuminatinglight beam and the wavelengths used in the calculation varies from 0.23to 0.850 microns with an uniform increment of 0.01 micron which resultsin 63 datapoints per signature curve. The light beam is assumed to be TEpolarized and is illuminated at an oblique angle of 76° from the normal.FIG. 3C is a plot of the mean squares error difference measurement as afunction of linewidth, between the signature generated for the gratinghaving the linewidth of 250 nm and other signatures obtained at otherlinewidths using 2-Θ scatterometry. FIG. 3C also shows plots of the meansquares error difference measurement as a function of linewidth, betweenthe signature generated for the grating having the linewidth of 250 nmand other signatures obtained at other linewidths, and using thepreferred embodiment of this invention over a full range of the spectrumas well as over ultraviolet (UV) and visual wavelength bands of the fullspectrum. As will be evident from FIG. 3C, the spectroscopicscatterometer of this invention is more sensitive than the 2-Θscatterometer. The mean square area difference for 1 nm linewidth (CD)sensitivity are shown by Tables 1 and 2 below. TABLE 1 MSE Different for1 nm CD Sensitivity CD (nm) Full Band UV Band Visual Band 2-Θ 250 0.03390.0528 0.0142 0.0051

TABLE 2 MSE Ratio With Respect to 2-Θ CD (nm) Full Band UV Band VisualBand 250 6.62 10.31 2.78

From FIG. 3C, it is also evident that the sensitivity may be higher ifonly data collected using radiation at a sub-band of the full spectrumis used for matching the signature. Thus, even though the spectrometer34 records the diffraction for the full range of wavelengths in thespectrum, sensitivity may be improved if only the diffraction atwavelengths in the ultraviolet (UV) band is used to construct themeasured signatures from the diffracting structure of 12 c and 12 c′.Such signatures are then matched to signatures in the databasecalculated for the UV band as well. From FIG. 3B, it is noted that eachof the curves is a function characterizing a particular signature of agrating. While in FIG. 3C, information in the ultraviolet band mayprovide higher sensitivity compared to the visual band or the full band,information in a different portion of the spectrum may provide bettersensitivity for gratings of other shapes and dimensions. All suchvariations are within the scope of the invention.

Another aspect of the invention is based on the observation that,instead of detecting the intensity of the zero, first or other order ofdiffraction from structure 12 c or 12 c′, the apparatus 10 of FIG. 1Amay be used to detect ellipsometric parameters of such order diffractionfrom the structure for determining one or more parameters of thediffracting structure. In other words, during the scatterometer mode,computer 40 controls polarizer 28 and analyzer 32 to cause relativerotation and motion between them, and system 10 is used for measuringellipsometric parameters such as tan (psi) and cos (delta) adds aplurality of wavelengths, such as at wavelengths in the spectrum ofradiation source 22. With either known or measured index or refractionand film thickness information of the one or more underlying filmsunderneath the structure 12 c or 12 c′, the MMME model method describedabove may be used to construct a database of tan (psi) and cos (delta)as functions of wavelength, as illustrated in FIGS. 4A and 4B,corresponding to different values of parameters of the structure 12 c or12 c′. Thus as shown in FIG. 4A, the model may be used to construct fivefunctions for tan (psi) as functions of wavelength at five differentlinewidths. FIG. 4B illustrates a similar plot for the ellipsometricparameter cos (delta). The nominal linewidth is 180 nanometers. Bymeasuring the two ellipsometric parameters of structure 12 c or 12 c′ bymeans of system 10, the measured functions may be compared to those inFIGS. 4A and 4B to find the best fit. The sensitivity in using theellipsometric parameters is illustrated in FIGS. 5. FIG. FIG. 5 is aplot of the correlation between the ellipsometric parameterscorresponding to the nominal 180 nanometer value and those correspondingto the remaining four line width values. Other than the above noteddifferences, in this aspect of the invention where ellipsometricparameters are used for determining characteristics of the structure 12c, 12 c′, the system 10 operates in a manner and shares the sameadvantages essentially as those described above for measuring intensityof diffraction in determining characteristics of the structure 12 c, 12c′. For some applications, measuring the ellipsometric parameters mayoffer higher sensitivity.

While the construction of database is illustrated above by reference tofunctions corresponding to different linewidths, it will be understoodthat similar functions may be constructed using the model for otherparameters of the structure 12 c or 12 c′, such as height or wall angleof the structure. Such and other variations are within the scope of theinvention.

While the invention has been described by reference to variousembodiments, it will be understood that different changes andmodifications may be made without departing from the scope of theinvention which is to be defined only by the appended claims and theirequivalents.

1. A method for measuring one or more parameters of a diffracting structure on an underlying structure, said underlying structure having a thickness and an optical index, comprising: providing an optical index and a film thickness of the underlying structure; constructing a reference database of one or more parameters related to said diffracting structure using said optical index and film thickness of the underlying structure; directing a beam of electromagnetic radiation at a plurality of wavelengths at said diffracting structure; detecting intensities or ellipsometric parameters of a diffraction at said plurality of wavelengths from said structure of said beam; and comparing said detected intensities or ellipsometric parameters to said database to determine said one or more parameters. 2.-75. (canceled) 